Gas analyzers are used for many measurement and monitoring functions in science, industry and medicine. In particular, gas spectrometry is becoming widely used in diagnostic instrumentation based on the use of breath tests for detecting a number of medical conditions present in patients. Descriptions of much breath test methodology and instrumentation are disclosed in PCT Publication No. W099/12471, entitled “Breath Test Analyzer” by D. Katzman and E. Carlebach, some of the inventors in the present application. Methods of constructing and operating gas analyzers such as are used in breath test instrumentation are disclosed in PCT Publication No. W099/14576, entitled “Isotopic Gas Analyzer” by I. Ben-Oren, L. Coleman, E. Carlebach, B. Giron and G. Levitsky, some of the inventors in the present application. Applications of some breath tests for detecting specific medical conditions are contained in patents issued to one of the inventors of the present application, namely U.S. Pat. No. 5,962,335 to D. Katzman on “Breath Test for Detection of Drug Metabolism”, and U.S. Pat. No. 5,944,670 to D. Katzman on “Breath test for the Detection of Bacterial Infection” and in allowed U.S. patent application Ser. No.08/805415, by D. Katzman on “Breath test for the Diagnosis of Helicobacter Pylori Infection in the Gastrointestinal Tract”. Each of the above documents is hereby incorporated by reference in its entirety.
Such breath tests are based on the ingestion of a marker substrate, which is cleaved by the specific bacteria or enzymic action being sought, or as a result of the metabolic function being tested, to produce marked by-products. These by-products are absorbed in the blood stream, and are exhaled in the patient's breath, where they are detected by means of the gas analyzer.
One well known method of marking such substrates is by substituting one of its component atoms with an isotopically enriched atom. Such substrates and their by-products are commonly called isotopically labeled. One atom commonly used in such test procedures is the non-radioactive carbon-13 atom, present in a ratio of about 1.1% of naturally occurring carbon. Using 13C as the tracer, the cleavage product produced in many such tests is 13CO2, which is absorbed in the bloodstream and exhaled in the patient's breath. The breath sample is analyzed, before and after taking this marker substrate, typically in a mass spectrometer or a non-dispersive infra-red spectrometer. Detected changes in the ratio of 13CO2 to 12CO2 may be used to provide information about the presence of the specific bacteria or enzymic action being sought, or as a measure of the metabolic function being tested.
Since the amount of CO2 arising from the process under test may be a very small proportion of the total CO2 production from all of the bodies' metabolic processes, the breath test instrumentation must be capable of detecting very small changes in the naturally occurring percentage of 13CO2 in the patient's breath. Typically, the instrument should be capable of detecting changes of a few parts per million in the level of 13CO2 in the patient's exhaled breath, where the whole 13CO2 content in the patient's exhaled breath is only of the order of a few hundred ppm. For this reason, the sensitivity, selectivity and stability of the gas analyzers used in such tests must be of the highest possible level to enable accurate and speedy results to be obtained.
Furthermore, since the instrument is intended to operate in a point-of-care environment, where there is generally no continuous technician presence, the instrument must have good self-diagnostic capabilities, to define whether it is in good operating condition and fit for use. For similar reasons, it should also have a level of self calibration capability, to correct any drift in calibration level revealed in such self-diagnostic tests or otherwise.
The use of the instrument in a point-of-care environment adds additional importance to the speed with which an accurate diagnosis can be given to the patient following the test. Consequently, to increase patient compliance, the methods used in the breath test for analyzing the results of the measurements in terms of meaningful diagnostic information should be designed to provide as conclusive and reliable a result in as short a time as possible. Furthermore, the execution of the test in the physician's office is greatly facilitated by the use of simple patient and substrate preparation procedures.
In order to maintain the reliability of such tests, it is necessary to ensure that the calibration of the gas analyzer is maintained at the correct level. For this reason, in order to ensure maintenance of the high accuracy levels required, many of the prior art instruments necessitate the performance of complex and time-consuming calibration procedures, some of which have to be laboratory performed, rather than user-performed in the field. Since the advent of compact and low cost breath test instrumentation is making breath testing a widely used medical office procedure, instead of a hospital or laboratory procedure, the need for simple, user-performed, periodic calibration checks is becoming of prime importance.
Furthermore, the breath exhaled by patients always contains a naturally high level of humidity, and in the case of intubated patients, could also contain a high level of moisture and other secretions. The presence of such extraneous fluids can severely affect the ability of the gas analyzer to accurately measure the sought-after gas. Furthermore, constant exposure to high levels of humidity can have an adverse effect on the component parts of the gas analyzer, and especially on the measuring sensor itself For these reasons, moisture and humidity filters are advisable to maintain the accuracy of the instrument. Since the operator may have a tendency to use the filters provided with the instrument beyond the recommended number of times, thereby impairing the accuracy of the measurement, it is important that means be adopted to ensure that the filtration unit is not used beyond its stated lifetime.
There therefore exists a need to ensure the maintenance of the accuracy of breath test instrumentation, both by means of regular mandated calibration checks, and by ensuring regular mandated changes of the moisture filter used with the instrument. Furthermore, there is a need for the calibration check procedure to be capable of simple and preferably semi-automatic execution by the user, rather than requiring the intervention of a technician, or shipment to a calibration laboratory.
The unique characteristics of the breath test analyzer described in the above mentioned PCT Publication No. WO 99/12471, are due in large measure to the use of electrode-less cold gas discharge infra-red lamp sources, as described in U.S. Pat. No. 5,300,859, entitled “IR-Radiation Source and Method for Producing Same” to S. Yatsiv et al., hereby incorporated by reference in its entirety. One of the important advantages of such lamp sources is that they emit very narrow spectral lines at discrete frequencies characteristic of the molecular rotational-vibrational to ground state transitions of the excited gas species contained in the lamp. This is achieved in a source which is sealed-off, is compact, has a good level of conversion efficiency from electrical to optical power, and has a long life compared with previously available sealed-off lamps sources.
The unique spectral properties and the narrowness of the emission lines of such lamps provides such gas analyzers with high levels of selectivity, sensitivity and stability, which are many times better than gas analyzers of similar complexity, which use lamp sources of alternative technologies, such as hot blackbody sources. The other advantages mentioned above enable the production of compact and cost effective instrumentation using such sources.
The lamp sources described in U.S. Pat. No. 5,300,859 have found particularly advantageous applications as sources of the CO2 spectral emission lines, for gas analysis of exhaled breath, to determine the levels of CO2 therein. Such CO2 sources have been used to great advantage in capnography and breath testing instrumentation.
In U.S. Pat. No. 5,300,859, there is a thorough discussion regarding the parameters affecting the lamp emission rise and decay time, efficiency, excitation and output, and the lamp lifetime as a function of chemical methods used to clean the lamp before sealing. On the other hand, the question of the spectral stability of the lamp source is not addressed. However, when used as a frequency selective source in NDIR spectroscopic applications, spectral stability may even be more important than the above parameters. Intensity changes over time can easily be monitored and corrected by using a reference path, since lamp intensity is a single valued quantity. On the other hand, spectral changes can not be easily monitored or corrected for, because of the huge amount of information contained in a spectrum. Changes in the lamp spectrum cause changes in the absorption cell absorption characteristics. If these changes are not known, then it is impossible to accurately measure gas concentrations using such lamp sources. There therefore exists a serious need for a method of maintaining a high level of spectral stability in electrode-less cold gas discharge infra-red lamp sources. A high level of spectral stability would make an important contribution to the maintenance of accurate calibration levels in breath test instruments using such lamps.
The disclosures of all publications mentioned in this section and in the other sections of the specification, are hereby incorporated by reference, each in its entirety.